1. Field of the Invention
This invention relates to the molding of foamed articles having a laminated surface consisting of a solid thermoplastic sheet, such as a cast acrylic plastic sheet. The thermoplastic sheet can be a sheet having a particularly desirable smooth surface with integral coloring, or one having particular inertness to agents which would attack the underlying foam in end-use applications. Such sheet is generally more expensive than the polyethylene and polystyrene customarily employed in the injection molding of dense foam articles and such sheet in thin form, for purposes of economy, has been advantageously reinforced by techniques known to the art, for example, by an applied fiberglass-polyester backing. The composite articles of my invention, made in accordance with the described process, are a laminate of a solid thermoplastic sheet with a dense foam backing. These laminates can be used for wash basins, shower stalls, bathtubs, and the like. Such composite articles can combine the advantage of the low cost of polyethylene and polystyrene dense foam with the superior surface characteristics of acrylic sheet, or other solid thermoplastic sheet. Articles made by my technique are obtained in near-finished form and require very little final finishing operation, thus effecting a cost savings in manufacture.
2. Description of the Prior Art
The dense foam molding operation useful in the practice of my invention is suitably carried out in accordance with techniques known to the art, for example, as set forth in U.S. Pat. No. 3,268,636 -- R. G. Angell. This patent discloses a process for preparing a molded foamed thermoplastic article having a high density shell and an integral lower density core. The articles produced by this process, however, have essentially the same chemical and visual characteristics throughout their cross-section, so that the high density shell will have properties very similar to that of the lower density core.
Various foamed items having surfaces prepared from different polymers from that of the core are known to the art. For example, U.S. Pat. No. 2,806,812 -- E. H. Merz discloses a method for the preparation of thermoplastic sheets having a resin foam integrally bonded thereto. These sheets are made by preparing a mold assembly into which foamable resin beads are placed, after which the beads are foamed, a sheet of thermoplastic resin is applied to the top surface of the mold cavity containing the foamed beads, and atmospheric pressure is used to force the thermoplastic sheet into pressured engagement with the face of the resin foam.
Another technique of the prior art, set forth in U.S. Pat. No. 2,959,508 -- D. L. Graham et al. discloses expansion into foam structure of expandable granular thermoplastic material in discrete particulate form, by mixing this material with a self-reacting exothermic substance and placing it in a mold where the exothermic material causes an expansion which fills the mold cavity. The mold cavity may be fitted with a mold form of another plastic material to prepare a composite article.
U.S. Pat. No. 3,090,078 -- A. M. Stackhouse discloses a process for foaming cores of structural panels in place. Reference is made to conventional assembling procedures used for paneling systems employing a foamed resin core in which a panel is fixed by an adhesive to the foamed resin. These procedures are described as expensive and time-consuming. The new disclosure is directed to a technique for foaming or expanding a resin between a pair of skin surfaces in situ to form a standardized panel by introducing a foamable resin between panels, held in a fixed relationship, through a formulating and mixing nozzle which is made to extend the length and breadth of a particular panel and which injects a metered amount of foam-forming material as it retreats from the panel cavity. This is a free-foaming technique making use of foamable resins of the type employing water as a vapor for blowing purposes.
U.S. Pat. No. 3,124,626 -- D. L. Graham et al. discloses a process for producing boat hulls of sandwich construction. In this technique, a male mold and a female mold are spaced apart from each other to form a mold cavity. Prefoamed thermoplastic expandable beads are introduced into this mold cavity as discrete particles following which a mixture of an epoxy resin curing agent is injected. A resulting chemical reaction develops exothermic heat which causes full expansion and setting of the thermoplastic beads together, with impregnation of the reinforcing material with the reacted epoxy resin.
U.S. Pat. No. 3,163,686 -- R. O. Dusel et al. discloses a method for forming composite plastic devices, described as an improvement over the prior art technique of covering exposed surfaces of expanded cellular bodies with a film of plastic to form a laminated structure. The invention of this patent involves first thermoforming a thermoplastic sheet to the general outer contour of a desired article. The thin-walled receptacle thus formed is then inverted and placed within a cavity of a mold and this cavity is filled with heat-expandable polystyrene beads, following which the beads are expanded by the application of heat, so that a sandwich construction article is obtained.
Patents such as those set forth above are illustrative of the techniques employed by the prior art in the production of laminated plastic articles and are typical of the technology known to the art. I have conceived a novel and improved technique for making dense foam-supported plastic articles which may have substantially more rigid structures than can be obtained by techniques such as set forth above and substantially greater densities, with accompanying greater strength, than obtainable by the expanded particle and free-foaming techniques used in the prior art relating to plastic core or backing materials.